Flotation is a well known process having the aim of extracting a metal from low-content ores by a stage of concentration. This stage comes before a subsequent treatment comprising the heat treatment (also known as smelting) or the leaching and the refining. This is in particular the case with ores of oxides and/or of sulphides of lead, of zinc, of copper, of silver, of gold, of molybdenum and of metals belonging to the platinum group: platinum, palladium, rhodium, ruthenium, iridium and osmium.
The ores include the compounds (such as oxides or sulphides) of the metals to be extracted (or metal compounds of value) in the form of crystals which are dispersed in a gangue composed of various impurities, in particular siliceous impurities. The ores are therefore, after extraction from the mine, crushed and then milled in a wet medium to give particles which are sufficiently fine to release the crystals of the desired compounds.
During flotation, the highly heterogeneous mixture, on the one hand of the crystals of the desired compounds and, on the other hand, of the gangue particles, is therefore introduced into water comprising appropriate additives, in particular flotation agents, also known as flotation collectors. Air is injected into the aqueous suspension thus obtained, placed in a suitable device (generally a flotation cell), so as to create bubbles which adhere to the crystals of the compounds (such as oxides and/or sulphides) comprising the desired metal. The adhesion of the bubbles to the said crystals is promoted by the action of the flotation agent or agents used. The crystals of the metal compounds then rise to the surface and are recovered in the form of a foam, also known as flotation concentrate. The gangue particles are recovered in the lower part of the flotation cell.
The flotation concentrate has a content of desired metal which is therefore considerably higher than that in the starting ore. This content depends on the initial content in the ore and on the selectivity of the flotation process. The amount of metal recovered in the form of flotation concentrate for its part depends on the yield of the process.
Flotation agents commonly employed in the industry for the extraction of metals include, for example, alkali metal xanthates with alkyl radicals having less than 6 carbon atoms, in particular potassium ethyl, amyl or isobutyl xanthate, mercapto-benzothiazoles, thiocarbamates, dithiocarbamates and dithiophosphates.
After a stage of drying by filtration, the flotation concentrates, for example in the case of copper, are subsequently introduced, for the heat treatment (or smelting) stage, into a furnace at temperatures which may exceed 1500° C. During this stage, the desired metal is separated in the molten state from the other substances, in particular from the impurities originating from the gangue of the ore, which have to be removed in the form of a slag.
It is thus understood, in particular for this reason, that it is very important on a practical level to obtain, on conclusion of the flotation stage, in addition to a high yield, a concentrate which has a content of the desired metal which is as high as possible, so as to facilitate the subsequent operations of treatment of the said concentrate and of final isolation of the metal. This technical advantage for flotation concentrates with a high content of desired metal is also reflected by the enhancement in value and the cost of such concentrates, sold by mining companies to companies in charge of the recovery of the metal and of its purification (or refining). This aspect of the metal content of the concentrate resulting from flotation is particularly critical in the case of platinum, the content of which in ores is extremely low and usually of the order of 2 to 15 ppm.
The use of mercaptans as flotation agent is already known.
U.S. Pat. No. 4,211,644 thus discloses the use, in the production of flotation concentrates, of a solution of n-dodecyl mercaptan in a polyglycol. However, this document teaches such a use in the production of flotation concentrates with an increased content of copper.
South African Patent ZA 8405787 discloses the use, as flotation collector, of a solution of tert-dodecyl mercaptan in cresylic acid. This document also discloses the application of such collectors in the treatment of copper ores.
In point of fact, the industry for the extraction of platinum, and more especially for the concentration of platinum ores by flotation, is, for the reasons indicated above, always looking for novel means which make it possible to improve the content of platinum in the flotation concentrates (or selectivity) and the yield of the flotation.